One of the future alternatives to current fossil-based transportation fuels has been centered on hydrogen gas (H2). Currently, H2 is the primary energy source of today's space exploration projects (e.g., as rocket propellant). It is also used in fuel cells that power a variety of machinery including automobiles. Furthermore, H2 is an important industrial commodity produced and used in many industries. For example, it is used for the reduction of metal oxides (e.g. iron ore), ammonia synthesis, and production of hydrochloric acid, methanol and higher alcohols, aldehydes, hydrogenation of various petroleum, coal, oil shale and edible oils, among others. However, H2 is a colorless, odorless gas, and is also a flammable gas with a lower explosive limit of about 4% in air. Therefore reliable H2 sensors are required to detect H2 leaks wherever H2 is produced, stored, or used.
To detect H2, sensors comprising a palladium alloy Schottky diode formed on a silicon substrate are known. These sensors are based on metal-oxide-semiconductor (MOS) technology that is used in the semiconductor industry. The gas sensing MOS structures comprise a H2-sensitive metal (palladium or its alloy) on a dielectric (e.g., an oxide) adherent to a semiconductor. This H2 sensor has been commercialized and exploited for detecting H2 leaks during pre-launches of space vehicles. Others have also used palladium or the like as a sensing element for detecting H2. A H2 sensor containing an array of micromachined cantilever beams coated with palladium/nickel has also been disclosed.
Semiconductors with wide band-gap (e.g. gallium nitride) have also been used to make diodes for H2 detection. One of the concerns for all of these types of sensors using palladium or the like is the requirement of a high operating temperature (greater than 200° C.) and further elevated temperatures (greater than 500° C.) to reactivate the sensing element, bringing about lengthy analysis. Another issue is sensitivity of the sensing element to unintended compounds that are commonly found in the atmosphere, including water vapor, various hydrocarbons, and various reducing gases such as carbon monoxide and hydrogen sulfide.
Although not conventionally used, chemochromic H2 sensors are known. Some chemochromic H2 sensors lack field stability and have a tendency to crack and peel and some can be washed off by precipitation and/or condensation. Moreover, some chemochromic H2 sensors do not show selectivity to H2. Thus, there remains a need for an improved, reliable and durable chemochromic H2 sensor for a variety of applications, including space, transportation, oil refineries, and chemical plants.